This invention relates to a method of making a mixture of a benzoyl halide and a benzal halide by reacting a benzotrihalide with a benzaldehyde. In particular, it relates to the use of a salt of zinc or copper, such as zinc acetate, to catalyze that reaction.
Typically, water is used to partially hydrolyze benzotrihalides and produce the corresponding benzoyl halide. Due in part to the reactivity of the product with water and the co-production of 2 equivalents of HCl, this process presents a number of processing difficulties, such as overhydrolysis, which results in the formation of the corresponding benzoic acid and the reaction of the benzoic acid with the product benzoyl halide to produce benzoic anhydride. Because the benzoic acid and anhydride have a low solubility, solids can form in the reaction mixture. The HCl generated reduces the efficiency of the still during the distillation of the product. Water and benzoyl halide can be carried into the reactor vent with the HCl by entrainment and can cause plugging by forming a benzoic acid.
These processing issues can be mitigated by reacting the benzotrihalide with a benzaldehyde rather than with water. That reaction can proceed in reasonably high yield when a strong Lewis acid, such as ferric chloride, is used. However, significant tar formation has been reported. The higher distillation temperatures required to isolate the product further increases tar formation and often significantly reduces any benefits gained over the water hydrolysis method. See "Oxygen-Chlorine Carbenoid Exchange Reaction Between Aromatic Aldehydes and Compounds Containing a Dichloromethyl Group," by B. F. Filimonov et al. in Zh. Obshch. Khim. (1980), 50(6), 1366-72 (zinc chloride was tried as a catalyst in the reaction of benzaldehyde with o-chloro benzal chloride, but the yield was low.); "Oxygen-Chlorine Carbenoid Exchange Between Aldehydes and Compounds Containing a Trichloromethyl Group," by B. F. Filimonov et al. in Zh. Obshch. Khim. (1979), 49(5), 1098-105; "Chlorination of Aromatic Aldehydes by Benzotrichloride in the Presence of Iron(III) Chloride," by B. F. Filimonov et al., Zh. Obshch. Khim. (1977), 47(7), 1670; "Kinetics and Mechanism of the Chlorination of Aromatic Aldehydes with Benzotrichloride," by G. F. Dvorko et al., Zh. Obshch. Khim. (1985), 55(8), 1828-35; and "Catalysis by Acids and Their Anhydrides of Oxygen-Chlorine Exchange Reactions Between Aryldichloromethanes and Aromatic Aldehydes," by G. F. Dvorko et al., Zh. Obshch, Khim. (1981), 51(9), 2067-75.
In addition, benzonitriles can be made by reacting a benzotrihalide with an ammonium halide in the presence of a catalytic amount of a benzoic acid. An intermediate amide is formed, which is dehydrated by the benzotrihalide to form the nitrile and a benzoyl halide. The benzoyl halide reacts with additional ammonium halide to form additional amide and continue the cycle (see U.S. Pat. No. 5,866,709). To avoid introducing other compounds into the product mixture, the benzoic acid selected as the catalyst should correspond to the isomer of the benzotrihalide being reacted. Unfortunately, the corresponding benzoic acid catalyst is often difficult to obtain and/or is expensive. Additionally, benzoic acids have high melting points and present solids handling problems in the production of nitriles.